Constitutive activation of NF-kappaB virally-associated lymphomas: Analysis of pro-survival pathways and therapeutic potential by pharmacological inhibition

NF-kappaB comprises a family of inducible transcription factors that serve as important regulators of cellular growth and survival. In Epstein-Barr virus (EBV) and human T-cell leukemia virus type-I (HTLV-I) associated lymphomas, constitutive activation of NF-kappaB by transforming viral genes is postulated to be a key event that mediates lymphomagenesis via regulation of genes necessary for cellular proliferation and survival. In this study we showed that Kaposi's sarcoma associated herpesvirus (KSHV) infected primary effusion lymphoma (PEL) cells also show constitutive activation of NF-kappaB characteristic of EBV and HTLV-I infected lymphoma cells. The exact role of KSHV in the transformation of PEL cells is unknown but like EBV and HTLV-1, KSHV encodes viral proteins that mimic cellular functions to promote the survival and proliferation of infected lymphocytes. Here we demonstrated that expression of KSHV-encoded FLIP in lymphocytes activated NF-kappaB and induced expression of anti-apoptotic proteins. Moreover, activation of NF-kappaB induced by vFLIP as well as the constitutive NF-kappaB activity in PEL cells was TRAF2-dependent strongly suggesting a role for vFLIP in the constitutive activation of NF-kappaB in KSHV infected lymphoma cells.; Further studies using pharmacological inhibition were performed to evaluate the importance of constitutive NF-kappaB activation in the survival of KSHV, EBV, and HTLV-1 infected lymphoma cells. Inhibition experiments demonstrated that Bay 11-7082, an irreversible inhibitor of IkappaBalpha phosphorylation, completely abrogated NF-kappaB activation and induced the apoptosis of these cells. Assessment of apoptosis induced in KSHV and EBV infected lymphoma cells following inhibition of NF-kappaB revealed down-regulation of specific anti-apoptotic, cell signaling, and growth factors common to both cell types. In these cells inhibition of NF-kappaB resulted in activation of both caspase-dependent and independent apoptotic mechanisms. These findings suggest that in KSHV and EBV infected lymphoma cells, constitutive activation of NF-kappaB is vital for the regulation of pro-survival factors that work cooperatively to protect cells from normal stress stimuli that induce apoptotic pathways. In vivo mouse models demonstrated that treatment of mice with Bay 11 significantly delayed the onset and development of PEL suggesting that inhibition of NF-kappaB, either alone or in conjunction with other treatment strategies, may be an effective treatment for KSHV and other virally associated lymphomas.